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Abstract:

There is provided an apparatus for measuring levels of a specified gas in
exhaled breath, the apparatus comprising a photoacoustic sensor for
providing a measurement representative of the level of the specified gas
in the exhaled air, wherein the photoacoustic sensor comprises a light
source that is modulated at a first frequency; a sound speed measurement
module for measuring the sound speed of the exhaled breath, wherein the
sound speed measurement module operates either at a second frequency that
is substantially different to the first frequency or in a pulsed mode;
wherein the first frequency of the modulated light source is adjusted
during exhalation in accordance with the measured speed of sound of the
exhaled breath.

Claims:

1. An apparatus (2) for measuring levels of a specified gas in exhaled
breath, the apparatus (2) comprising: a photoacoustic sensor (10) for
providing a measurement representative of the level of the specified gas
in the exhaled air, wherein the photoacoustic sensor (10) comprises a
light source (20) that is modulated at a first frequency; a sound speed
measurement module (6) for measuring the sound speed of the exhaled
breath, wherein the sound speed measurement module (6) operates either at
a second frequency that is substantially different to the first frequency
or in a pulsed mode; wherein the first frequency of the modulated light
source (20) is adjusted during exhalation in accordance with the measured
speed of sound of the exhaled breath.

2. An apparatus (2) as claimed in claim 1, wherein the photoacoustic
sensor (10) comprises a resonator chamber (18) that is operated in a
plane wave mode, and wherein the first frequency of the modulated light
source (20) is adjusted in accordance with the measured speed of sound of
the exhaled breath and the acoustic mode of the resonator chamber (18).

3. An apparatus (2) as claimed in claim 1, wherein the sound speed
measurement module (6) comprises a pair of transducers (12, 14), each
transducer (12, 14) being arranged to transmit an ultrasonic pulse
through the exhaled breath to the other transducer (12, 14).

5. An apparatus (2) as claimed in claim 4, wherein the sound speed
measurement module (6) operates at a second frequency that is
approximately half of the first frequency.

6. An apparatus (2) as claimed in claim 1, wherein the sound speed
measurement module (6) comprises: a pair of transducers (12, 14)arranged
on opposite sides of a passage (16) through which the exhaled breath
passes, each transducer (12, 14) being arranged to transmit an ultrasonic
pulse through the exhaled breath to the other transducer (12, 14) and to
provide a signal indicative of the transit time of the ultrasonic pulse;
wherein the pair of transducers (12, 14) are arranged such that each
transducer (12, 14) transmits their respective ultrasonic pulses along an
axis that is at a non-zero angle with respect to a plane that is
perpendicular to the direction in which the exhaled breath flows; and a
processor (24) for determining the sound speed in the exhaled breath from
the sum of the transit times, and the exhalation flow from the difference
of the transit times of the ultrasonic pulses through the exhaled breath;
wherein the apparatus (2) is configured to provide a measurement of the
specified gas and the exhalation flow.

7. An apparatus (2) as claimed in claim 1, wherein sound speed
measurement module (6) comprises: a pair of transducers (12, 14) arranged
on opposite sides of a passage through which the exhaled breath passes,
each transducer (12, 14) being arranged to transmit an ultrasonic pulse
through the exhaled breath to the other transducer (12, 14) and to
provide a signal indicative of the transit time of the ultrasonic pulse;
a temperature sensor (21) for determining the temperature of the gas
close to the transducers (12, 14); and a processor (24) for determining
the sound speed in the exhaled breath from the transit time of the
ultrasonic pulse, and a molar mass from the transit time and temperature;
and wherein the apparatus (2) is configured to provide a measurement of
the specified gas and the molar mass.

8. An apparatus (2) as claimed in claim 1, wherein the specified gas is
nitric oxide.

9. An apparatus (2) as claimed in claim 8, wherein the apparatus (2)
further comprises a converter (8) for converting nitric oxide in the
exhaled breath to nitrogen dioxide to provide a measurement sample, and
wherein the photoacoustic sensor (10) measures the levels of nitrogen
dioxide in the measurement sample, the measured levels of nitrogen
dioxide corresponding to the levels of nitric oxide in the exhaled
breath.

10. An apparatus (2) as claimed in claim 8, wherein the apparatus (2)
further comprises a processing unit (24) for determining one or more
parameters describing the nitric oxide production in the airways based on
the exhaled nitric oxide and flow patterns during one or more
exhalations.

11. An apparatus (2) as claimed in claim 8, wherein the apparatus (2)
further comprises a processing unit (24) for determining one or more
parameters describing the nitric oxide production in the airways based on
the exhaled nitric oxide, flow and molar mass patterns during one or more
exhalations.

12. A method of measuring levels of a specified gas in exhaled breath,
the method comprising: measuring the speed of sound of the exhaled breath
(107); adjusting a modulation frequency of a light source in a
photoacoustic sensor during exhalation in accordance with the speed of
sound of the exhaled breath (115); and using the photoacoustic sensor to
provide a measurement representative of the level of the specified gas in
the exhaled breath (117); wherein the speed of sound is measured either
at a frequency that is substantially different to the modulation
frequency or in a pulsed mode.

13. A method as claimed in claim 12, wherein the photoacoustic sensor
provides a measurement representative of the level of the specified gas
in the exhaled breath by: passing the laser beam through a measurement
sample of the exhaled breath that is contained in a resonance chamber of
the photoacoustic sensor, the laser beam having a wavelength in the
absorption range of the specified gas (115); and measuring the sound
generated from the laser beam passing through the measurement sample of
the exhaled breath to provide the measurement representative of the
specified gas in the exhaled breath (117).

14. A method as claimed in claim 12, wherein the specified gas is nitric
oxide and wherein the method further comprises the step of: passing a
sample of the exhaled breath through a nitric oxide to nitrogen dioxide
converter to generate a measurement sample (111); wherein the
photoacoustic sensor measures the level of nitrogen dioxide in the
measurement sample to provide the measurement representative of the level
of nitric oxide in the exhaled breath.

Description:

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a method and apparatus for measuring the
levels of a specific gas in exhaled breath.

BACKGROUND OF THE INVENTION

[0002] It is known that the concentration of nitric oxide (NO) in exhaled
air can be used as an indicator of various pathological conditions. For
instance, the concentration of exhaled nitric oxide (eNO) is a
non-invasive marker for airway inflammation. Inflammation of the airways
is typically present in people with asthma and monitoring for high
concentrations of eNO can be used in a test which is useful in
identifying asthma.

[0003] Furthermore, measurements of eNO can be used for monitoring the
effectiveness of inhaled corticosteroids (ICS) and in anti-inflammatory
asthma management to titrate ICS dosage.

[0004] The standardized method of measuring eNO requires a single
exhalation test at a fixed flow rate of 50 ml/s at an overpressure of at
least 5 cm H2O. The exhalation test requires a constant exhalation
flow for a given period of at least 10 seconds, and is not simple to
perform by those with breathing difficulties or by young children.
Therefore, conventional devices make use of visual and acoustic feedback
signals to guide the user through the test successfully. Commercially
available systems from Aerocrine and Apieron have received U.S. FDA
approval for standardized eNO measurements in children aged 7-18 years
and adults under supervision of a trained operator in a physician's
office. No FDA-approved system for young children is currently available.

[0005] It is clear that a more straightforward and natural breathing
procedure (for example tidal breathing) would be more preferable for
young children and for non-professional (i.e. home) use.

[0006] It has been proposed in EP application no. 09166814.5 to measure
the flow rate and eNO during an exhalation and subsequently analyze the
measured data using a model that describes the generation and transport
of NO in the airway system. In this way, flow-independent parameters can
be deduced from tidal breathing patterns and if necessary, the value at
50 ml/s used in the standardized method can be derived.

[0007] An apparatus has been developed that measures eNO with a
NO-to-NO2 (nitric oxide to nitrogen dioxide) converter and a
photoacoustic sensor for NO2. The latter has been described in
"Relaxation effects and high sensitivity photoacoustic detection of
NO2 with a blue laser diode" by Kalkman and Van Kesteren in Applied
Physics B 90 (2008) p 197-200. This apparatus enables, in combination
with a NO-to-NO2 converter, a detection limit of NO in the low
parts-per-billion (ppb) range and a real-time measurement of the NO
concentration as required for tidal breathing, but an acoustic resonator
with a high quality factor is required as part of the photoacoustic
sensor in order to reach this detection limit.

[0008] However, during tidal breathing, the concentrations of O2 and
CO2 in the exhaled breath change and this results in a change in the
speed of sound of the exhaled air.

[0009] The related shift of the resonance frequency of the acoustic
resonator leads to a variation in the response to NO during the
exhalation.

[0010] In a paper entitled "Photoacoustic spectrometer for measuring light
absorption by aerosol: instrument description" by Arnott et al.
[Atmospheric Environment 33 (1999) p 2845-2852] a photoacoustic
spectrometer is described which incorporates a piezoelectric disk for
sound generation that can be used to determine the resonance frequency of
the photoacoustic cell. This spectrometer could either be operated in a
mode to determine the resonance frequency with the piezoelectric disk or
be operated in a photoacoustic gas sensing mode with the light source
being modulated at a fixed frequency and the piezoelectric disk switched
off. For environmental air with a slowly varying composition and
temperature this approach works satisfactory. However, as with the
previously-described apparatus, this photoacoustic spectrometer cannot
adapt to the shift of the resonance frequency that occurs during
exhalation due to changes in concentration of O2 and CO2.

[0011] In principle the photoacoustic sensor can be operated at various
modes of the acoustic resonator and non-interfering longitudinal and
transverse modes can be chosen for photoacoustic sensing and resonance
tracking. In practice, the involvement of longitudinal as well as
transverse modes leads to large resonator sizes and a significant loss of
sensitivity.

SUMMARY OF THE INVENTION

[0012] Therefore, there is a need for an improved apparatus that overcomes
this problem with measurements of NO and other specific gases in exhaled
breath. Furthermore, it would be advantageous if the sensor module for NO
detection in exhaled breath provides the NO concentration in combination
with flow and molar mass of the gas mixture to enable an accurate
analysis of the NO production and transport in the airways.

[0013] There is therefore provided an apparatus for measuring levels of a
specified gas in exhaled breath, the apparatus comprising a photoacoustic
sensor for providing a measurement representative of the level of the
specified gas in the exhaled air, wherein the photoacoustic sensor
comprises a light source that is modulated at a first frequency; a sound
speed measurement module for measuring the sound speed of the exhaled
breath; wherein the first frequency of the modulated light source is
adjusted during exhalation in accordance with the measured speed of sound
of the exhaled breath.

[0014] According to a second aspect of the invention there is provided a
method of measuring levels of a specified gas in exhaled breath, the
method comprising measuring the speed of sound of the exhaled breath;
adjusting a modulation frequency of a light source in a photoacoustic
sensor during exhalation in accordance with the speed of sound of the
exhaled breath; and using the photoacoustic sensor to provide a
measurement representative of the level of the specified gas in the
exhaled breath.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments of the invention will now be described in more detail,
by way of example only, with reference to the following drawings, in
which:

[0016] FIG. 1 is a block diagram of an apparatus in accordance with a
first embodiment of the invention;

[0017] FIG. 2 is a block diagram of an apparatus in accordance with a
second embodiment of the invention;

[0018]FIG. 3 is a block diagram of an apparatus in accordance with a
third embodiment of the invention; and

[0019]FIG. 4 is a flow chart illustrating a method in accordance with the
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A first embodiment of the apparatus for measuring exhaled nitric
oxide (NO) levels according to the invention is shown in FIG. 1. The
apparatus 2 comprises a breathing tube or mask 4 into which a patient
exhales, a sound speed measurement module 6 located in the breathing tube
4, a humidity reduction unit 7, a nitric oxide to nitrogen dioxide
(NO2) converter 8 for converting the nitric oxide in a small
measurement sample of the exhaled breath into nitrogen dioxide, and a
photoacoustic sensor 10 that measures the level of nitrogen dioxide in
the measurement sample.

[0021] The sound speed measurement module 6 preferably operates in the
ultrasound frequency range (i.e. typically between 20 kHz and 200 kHz)
and measures transit-times of sound pulses between a pair of transducers
12, 14 along and against the direction of flow of the exhaled breath. The
flow is derived from the difference in transit times and is independent
of the gas composition (i.e. is not affected by changes in concentration
of oxygen and carbon dioxide). During tidal breathing, the sound speed
measurement module 6 allows the apparatus 2 to take account of the flow
dependent NO production in the airways of the patient.

[0022] The sum of the ultrasonic pulse transit times between the two
transducers 12, 14 is used to derive the speed of sound of the exhaled
gas mixture, and this information is used to adjust a frequency of the
photoacoustic sensor 10. This significantly improves the accuracy of eNO
detection during tidal breathing. In combination with the temperature of
the exhaled breath which is reasonably well known, the molar mass of the
exhaled breath can also be derived from the sum of the ultrasonic pulse
transit times. For a higher accuracy a temperature sensor 21 can be
incorporated in the breathing tube 4. The shape of the molar mass pattern
during exhalation, being similar to a capnogram, provides information on
CO2/O2 gas exchange and obstruction in the respiratory tract
which can be taken into account in the analysis of the flow dependent NO
production in the airways as described in EP application no. 07111132.2.

[0023] In the illustrated embodiment, the pair of transducers 12, 14 are
arranged at a non-zero angle to a plane that is perpendicular to the
direction in which the exhaled air passes through the apparatus 2. In
other words, the transducers 12, 14 emit ultrasonic pulses at an angle
across the direction of flow of exhaled breath.

[0024] Each of the transducers 12, 14 operate as a transmitter as well as
a receiver to enable the measurement of the transit times of short
ultrasound pulses in both directions across the exhaled air flow.

[0025] Part of the exhaled breath passing through the tube 4 is separated
into a side stream 16 to provide the measurement sample which is passed
to the humidity reduction unit 7 and NO to NO2 converter 8.
Depending on the material and composition of the converter, typically 80
to 100% of the NO is converted into NO2 After the converter 8, the
measurement sample (with NO2) is passed into the photoacoustic
sensor 10 which determines the NO2 concentration.

[0026] The photoacoustic sensor 10 comprises a resonator tube 18 that
operates in a longitudinal plane wave mode and has a resonant frequency,
fr, a laser 20 that generates a laser beam that is passed through a
window 19 and buffer volume 15 to the measurement sample in the resonator
tube 18. A microphone 22 records the intensity of the sound generated by
the laser beam passing through the measurement sample. The laser 20
generates a laser beam that has a wavelength within the absorption range
of NO2, and the intensity of the laser beam is modulated at a
frequency that substantially corresponds to the resonant frequency
fr of the resonator tube 18. The periodic absorption of optical
energy and subsequent release of thermal energy leads to pressure
variations that are picked-up by the microphone 22. Synchronous detection
of the microphone signal at the laser modulation frequency results in a
signal proportional to the NO2 concentration. The optimal dimensions
and thereby the resonant frequency fr of the resonator tube 18
depends on many factors, such as the relaxation dynamics of the gas being
detected, spectral noise behavior of the microphone 22, interfering noise
sources, etc. Typically, the resonant frequency fr is a frequency
from a few hundred Hz to a few kHz. Where the photoacoustic sensor 10 is
used for detecting levels of NO2, the resonance frequency can for
instance be at 5 kHz.

[0027] Preferably, the sound speed measurement module 6 operates at a
frequency that is substantially different to the resonant frequency of
the resonator tube 18 (or the sound speed measurement module 6 operates
in a pulsed mode).

[0028] As indicated in the Background section, the spectral bandwidth of a
photoacoustic resonator is described by a quality factor, and the quality
factor is equal to the resonant frequency, fr, divided by the
bandwidth. Typical quality factors for photoacoustic resonators are in
the range 5 to 50. In one embodiment, the frequency at which the sound
speed measurement module 6 operates is substantially different to the
resonant frequency of the resonator tube 18 if the frequency at which the
sound speed measurement module 6 operates deviates by more than 5 times
the bandwidth from the resonant frequency, fr, of the photoacoustic
sensor 10.

[0029] This allows the measurements of the sound speed to be made at the
same time, or substantially at the same time, that the photoacoustic
sensor 10 analyses the measurement sample (i.e. both measurements can be
carried out during normal breathing by the subject). For example, the
frequency used by the sound speed measurement module 6 can be in the
range of tens of kHz to MHz, as these high frequencies enable accurate
transit time measurements over small distances and do not interfere with
acoustic resonance that is at a few kHz or few hundred Hz.

[0030] Where the sound speed measurement module 6 operates in a pulsed
mode, a typical pulse could be two cycles of a 100 kHz wave. Again, for
NO2 detection, the resonant frequency can be 5 kHz.

[0031] As described above in the Background section, the quality factor of
the resonator tube 18 is preferably high to enable accurate detection of
NO2 concentrations in the low parts-per-billion (ppb) range.
However, the high quality factor makes the apparatus 2 more sensitive to
variations in the main constituents of exhaled breath like O2 and
CO2, and these do vary during tidal breathing.

[0032] Therefore, a processor 24 is provided that is connected to the
sound speed measurement module 6 to receive the pulse transit times,
derive from the difference in the ultrasound transit times from each of
the transducers 12, 14 to the other the magnitude of the exhalation flow
and from the sum of the transit times the sound speed for the exhaled
breath mixture. The exhalation flow is used in combination with the
exhaled NO level to derive one or more flow-independent parameters
describing the NO production and transport in the airways.

[0033] The processor 24 uses the sound speed for the exhaled breath to
derive a control signal for the laser 20 in the photoacoustic sensor 10.
The control signal is used to fine tune the modulation frequency of the
laser 20 in the photoacoustic sensor 10. Preferably the modulation
frequency of the laser 20 in the photoacoustic sensor 10 is adjusted
continuously or regularly during the breathing cycle. Changes in sound
speed and resonance frequency are coupled because the resonator tube 18
has a fixed length. So for an optimal performance of the photoacoustic
sensor the resonance frequency and thus the laser modulation frequency
have to be adjusted to changes in the gas composition.

[0034] It will be appreciated that there will be differences in the
temperature and humidity of the exhaled air in the breathing tube 4 close
to the sound speed measurement module 6 and the exhaled air in the
measurement sample in the photoacoustic sensor 10. Furthermore, the sound
speed measured in the ultrasound frequency range can deviate slightly
from the sound speed relevant at the resonance frequency of the
photoacoustic sensor 10. In a fixed apparatus 2, i.e. where the distance
between and the respective arrangement of the sound speed measurement
module 6 and the photoacoustic sensor 10 is fixed, the temperature
difference can be determined separately (perhaps in a calibration test)
and a correction constant can be set in the processor 24 to compensate
for the aforementioned effects. In an alternative approach, temperature
sensors 21 and 23 are incorporated in the breathing tube 4 and
photoacoustic sensor 10 respectively. Because the dependence of sound
speed on temperature is known, the sound speed can be accurately
compensated for the temperature difference. The effect of the humidity
difference on the sound speed is much smaller than the effect of the
temperature difference so generally it will not be necessary to
compensate for this effect. The humidity of the exhaled air is close to
saturation while the humidity reduction unit 7 reduces the humidity in a
known way so the humidity difference is known and the sound speed can be
compensated for this difference if necessary. Differences in sound speed
due to differences in sound frequency applied in the photoacoustic sensor
10 and in the sound speed measurement module 6 will generally be
negligible when the ultrasound sensor is operated at frequencies below
100 kHz.

[0035] Moreover, depending on the specific length of and flow in the side
stream 16, there can be a small time delay between the sound speed
measurement module 6 measurement and the photoacoustic sensor 10
measurement. The processor 24 can be configured to take this time delay
into account.

[0036] In an alternative embodiment of the invention shown in FIG. 2, the
sound speed measurement module 6 based on an ultrasonic transit time
measurement and the photoacoustic sensor 10 can be combined into a single
device. In particular, the transit time sensor 6 can be incorporated into
the buffer chamber 15 which enables accurate detection of the sound speed
from the measurement sample. This combination is possible because the
sound speed measurement module 6 operates at frequencies in the tens of
kHz to MHz range, and these high frequencies enable accurate transit time
measurements over small distances and do not interfere with the acoustic
resonance at a few kHz or few hundred Hz (i.e. as in the embodiment
above, the sound speed measurement module 6 operates in a frequency range
that is substantially different to the frequency range in which the
resonant frequency of the resonator tube 18 lies (or the sound speed
measurement module 6 operates in a pulsed mode)).

[0037] A third embodiment is shown in FIG. 3. The photoacoustic cell is
made of a transparent material, for instance glass, has a resonator
chamber 18 operating at a resonance frequency fr with acoustic
pressure nodes around the position of the windows 19 and a pressure
antinodes at the position of the microphone 22 and in the middle between
the windows. The sound speed measurement module consists of two
anti-phase modulated light sources 31 and 32 (for instance two LED's) and
the microphone 22 to pick up the generated sound signal. The sound is
generated by partial absorption of the light in the wall of the resonator
chamber 18 and thermal coupling with the gas inside the resonator chamber
18. The sound speed measurement module makes use of a special
longitudinal resonance of the photoacoustic cell at a frequency close to
1/2 fr. This mode does not interfere with the mode at fr
because it is only excited by anti-phase pressure antinodes excited by
the anti-phase modulated light sources 31 and 32. The laser beam with a
substantially constant power distribution over the cell length will not
excite this mode. The sound speed is derived from the modulation
frequency of light sources 31, 32 yielding a maximum microphone signal. A
processor 24 calculates on basis of this signal the resonance frequency
fr and adjusts the laser modulation frequency accordingly. The
wavelength of the LED sources 31, 32 is not critical because of the
generally broad absorption feature of the photoacoustic cell wall. The
sound speed derived from the optimum modulation frequency of the light
sources 31, 32 together with the temperature of the gas mixture
determined by temperature sensor 23 will yield the molar mass of the gas
sample in the photoacoustic cell. In one particular implementation of
this embodiment, the resonant frequency for NO2 detection is 5 kHz,
the frequency at which the sound speed measurement module 6 operates at a
frequency of 2.5 kHz, and the quality factor of the photoacoustic sensor
10 is 20.

[0038] In implementations of the invention where the apparatus 2 is for
use in detecting levels of specific gases other than NO, the laser 20 is
configured to generate a laser beam having a wavelength within the
absorption range of that specific gas. Moreover, the converter 8 can be
modified or omitted as appropriate depending on the specific gas in the
exhaled breath to be measured.

[0039] A method of determining nitric oxide levels in exhaled breath
according to the invention is shown in FIG. 4. In step 101, an exhaled
breath sample passes through the sound speed measurement module 6. Then,
in step 103 the time is determined from the start of the measurement
procedure.

[0040] Then, in step 105, the flow rate of the exhaled breath is measured,
preferably using an ultrasonic flow sensor 6 as described above.

[0041] In step 107, the speed of sound of the exhaled breath is
determined.

[0042] In step 109, the molar mass is determined from the transit times
and temperature of the exhaled breath sample.

[0043] In step 111, a measurement sample of the exhaled breath sample is
passed through a humidity reduction unit 7, a NO to NO2 converter 8
and the resulting sample is passed into a resonator chamber 18 of a
photoacoustic sensor 10 (step 113).

[0044] In step 115, a laser beam is generated having a wavelength within
the absorption range of NO2 and the intensity of the laser beam is
modulated at a frequency that substantially corresponds to the resonance
frequency of the photoacoustic sensor 10 which is determined from the
speed of sound of the breath sample, taking into account correction
factors to the sound speed and gas transport time in the side stream
sample line 16.

[0045] In step 117, the NO level in the exhaled breath is determined from
the measured sound using the instrument constants for the microphone
output per concentration unit of NO2 and conversion ratio of
NO2 to NO for the converter 8.

[0046] Finally, in step 119 an output data set is created combining the NO
concentration and corresponding flow, molar mass and time from the start
of the measurement.

[0047] The method then returns to step 101 and repeats the data generation
for a time frame. It will be appreciated that as the method is for use
during tidal breathing, the method therefore repeats continuously.

[0048] All the output data sets for one or multiple exhalations generated
according to this method form the inputs for an analysis module that
extracts one or more parameters describing the NO production and gas
transport in the airways. These in turn provide a measure of the airway
inflammation either or not in combination with information on an
obstruction in the airways.

[0049] There is therefore provided an improved apparatus for measuring
levels of a specific gas, and particularly nitric oxide, in exhaled
breath.

[0050] While the invention has been illustrated and described in detail in
the drawings and foregoing description, such illustration and description
are to be considered illustrative or exemplary and not restrictive; the
invention is not limited to the disclosed embodiments.

[0051] Variations to the disclosed embodiments can be understood and
effected by those skilled in the art in practicing the claimed invention,
from a study of the drawings, the disclosure and the appended claims. In
the claims, the word "comprising" does not exclude other elements or
steps, and the indefinite article "a" or "an" does not exclude a
plurality. A single processor or other unit may fulfill the functions of
several items recited in the claims. The mere fact that certain measures
are recited in mutually different dependent claims does not indicate that
a combination of these measures cannot be used to advantage. A computer
program may be stored/distributed on a suitable medium, such as an
optical storage medium or a solid-state medium supplied together with or
as part of other hardware, but may also be distributed in other forms,
such as via the Internet or other wired or wireless telecommunication
systems. Any reference signs in the claims should not be construed as
limiting the scope.